In LTE (Long Term Evolution) and LTE-Advanced radio communication systems, as shown in FIG. 1, there is a case where, inside a Macro Cell formed by Macro eNB (macro base station) 10 of a certain frequency band, a Pico Cell formed by Pico eNB (pico base station) 20 of the same frequency band is installed.
Recently, in LTE-Advanced (3GPP LTE Rel-10), ABS (Almost Blank Subframe) has been introduced in order to reduce inter-cell interference between a Macro Cell and a Pico Cell installed in the Macro Cell in the same frequency band (Non-patent Document 1).
The ABS is an almost blank subframe, and Macro eNB10 performs transmission by replacing several normal subframes with ABSs. It should be noted that, at least, PDSCH (PHysical Downlink Shared Channel) is not transmitted by ABSs.
Incidentally, in FIG. 1, UE (User Equipment: mobile station) 30-1 that resides in the Pico Cell calculates CQI (Channel Quality Indicator) in the Pico Cell, which is the downlink channel quality information, and reports the calculated CQI value to Pico eNB20. UEs 30-1 and 30-2 that reside in the Macro Cell calculate CQI in the Macro Cell and report the calculated CQI value to Macro eNB10.
Macro eNB10 and Pico eNB20, based on the CQI value reported from each UE, allocate radio resources for downlink signals such as PDSCH, to the UE.
When UE30-1 is UE that supports Rel-10 ABS and can designate two types of subframes as the subframe on which CQI measurement is performed (which will be referred to hereinbelow as Rel-10 UE), Pico eNB20 designates two types of subframes on which CQI measurement should be performed, to Rel-10 UE30-1. As result, Rel-10 UE30-1 calculates two types of CQIs, CQI on the subframes that receive weak interference, and CQI on the subframes that receive strong interference, from the Macro Cell, as the CQI of the subframes that Pico eNB20 transmitted to the Pico Cell.
Specifically, in the example of FIG. 2, subframes #1, #3, #5 and #9 that Macro eNB10 transmitted to the Macro Cell are ABSs so that the interference that the Pico Cell receives from the Macro Cell is weak, while subframes #0, #2, #4, #6, #7 and #8 are normal subframes so that the interference that the Pico Cell receives from the Macro Cell is strong. In this case, Rel-10 UE30-1 calculates two types of CQIs, for subframes #1, #3, #5 and #9 and subframes #0, #2, #4, #6, #7 and #8 that Pico eNB20 transmitted to the Pico Cell. Here, the subframes for performing CQI measurement, designated by Pico eNB20 to Rel-10 UE30-1 may also be part of subframes #1, #3, #5 and #9 and part of subframes #0, #2, #4, #6, #7 and #8. Rel-10 UE30-1 calculates two types of CQIs based on the designated two types of subframes.
In this case, Pico eNB20, using only the CQI value that is calculated based on subframes #1, #3, #5 and #9 that receive weak interference from the Macro Cell, or part of the subframes, from among the two types of CQI values reported from Rel-10 UE30-1, can allocate the radio resources for downlink signals in Macro Cell's ABSs, that is to say subframes #1, #3, #5 and #9, to Rel-10 UE30-1.
With this arrangement, Pico eNB20 can allocate optimal radio resources for downlink signals to Rel-10 UE30-1 in the subframes that receive weak interference from the Macro Cell, based on the CQI value of the subframes.
On the other hand, when UE30-1 that resides in the Pico Cell does not support Rel-10 ABSs, and thus can only designate a single type of subframes for performing CQI measurement (which will be referred to hereinbelow as Rel-8/9 UE), Pico eNB20 can only designate a single type of subframes for performing CQI measurement, for Rel-8/9 UE30-1. As a result, Rel-8/9 UE30-1 calculates one type of CQI as the CQI of the subframes that Pico eNB20 transmitted to the Pico Cell.
Specifically, in the example of FIG. 3, subframes #1, #3, #5 and #9 that Macro eNB10 transmitted to the Macro Cell are ABSs so that the Macro Cell's interference with the Pico Cell is weak, while subframes #0, #2, #4, #6, #7 and #8 are normal subframes so that the Macro Cell's interference with the Pico Cell is strong. Despite that fact, Rel-8/9 UE30-1 calculates only one type of CQI for all the subframes #0 to #9 without considering the difference in interference from the Macro Cell between the two types of subframes.
In this case, Pico eNB20 is unable to take into account the interference from the Macro Cell in allocating radio resources for downlink signals to Rel-8/9 UE30-1, and thus performs radio resource allocation for downlink signals to all the subframes using the single type of CQI value reported by Rel-8/9 UE30-1.
Herein, in general, the single type of CQI value reported by Rel-8/9 UE30-1 is calculated based on both the subframes in which the Pico Cell receives strong interference from the Macro Cell and the subframes in which the Pico Cell receives weak interference from the Macro Cell. Accordingly, the CQI value becomes smaller than the CQI value reported from a Rel-10 UE that resides at the same place because the Rel-10 UE performs calculation based on only the subframes in which the Pico Cell receives weak interference from the Macro Cell.
As a result, Pico eNB20 determines that the CQI of Rel-8/9 UE30-1 as to the subframes in which the Pico Cell receives weak interference from the Macro Cell is inferior to the CQI of the Rel-10 UE that resides at the same place, hence allocates in response to that CQI, a lower order Modulation Scheme (e.g., allocates QPSK instead of 16 QAM) or a lower order Coding Rate to Rel-8/9 UE30-1. Therefore, Rel-8/9 UE30-1 consumes more radio resources for Pico Cell downlink signals than Rel-10 UE that resides at the same place, hence causing a reduction of the capacity of the radio communication system.
Here, though in the examples shown in FIGS. 1 to 3, it is assumed that Macro eNB10 alone transmits ABSs, it may be considered that Pico eNB20, in addition to Macro eNB10, also transmits ABSs.
Now, a case where not only Macro eNB10 but also Pico eNB20 transmits ABSs will be considered in the radio communication system shown in FIG. 4.
Herein, in FIG. 4, UE30-3 that resides in the Pico Cell calculates CQI in the Pico Cell, and reports the calculated CQI value to Pico eNB20. UE30-4 that resides in the Macro Cell calculates CQI in the Macro Cell and reports the calculated CQI value to Macro eNB10.
When UE30-3 that resides in the Pico Cell is a Rel-10 UE, Pico eNB20 designates two types of subframes on which CQI measurement should be performed, to Rel-10 UE30-3. As result, Rel-10 UE30-3 calculates two types of CQIs, CQI on the subframes that receive weak interference, and CQI on the subframes that receive strong interference, from the Macro Cell, as the CQI of the subframes that Pico eNB20 transmitted to the Pico Cell.
Specifically, in the example of FIG. 5, subframes #1, #3, #5 and #9 that Macro eNB10 transmitted to the Macro Cell are ABSs so that the interference that the Pico Cell receives from the Macro Cell is weak, while subframes #0, #2, #4, #6, #7 and #8 are normal subframes so that the interference that the Pico Cell receives from the Macro Cell is strong. At this stage, Rel-10 UE30-3 calculates two types of CQIs, for subframes #1, #3, #5 and #9 and subframes #0, #2, #4, #6, #7 and #8 that Pico eNB20 transmitted to the Pico Cell. Here, subframes for performing CQI measurement, designated by Pico eNB20 to Rel-10 UE30-3 may also be part of subframes #1, #3, #5 and #9 and part of subframes #0, #2, #4, #6, #7 and #8. Rel-10 UE30-3 calculates two types of CQIs based on the designated two types of subframes.
In this case, Pico eNB20, using only the CQI value that is calculated based on subframes #1, #3, #5 and #9 that receive weak interference from the Macro Cell, or part of the subframes, among the two types of CQI values reported from Rel-10 UE30-3, can allocate the radio resources for downlink signals in Macro Cell's ABSs, that is to say subframes #1, #3, #5 and #9, to Rel-10 UE30-3.
With this arrangement, Pico eNB20 is able to allocate optimal radio resources for downlink signals to Rel-10 UE30-3 in the subframes that receive weak interference from the Macro Cell, based on the CQI value of the subframes.
When UE30-4 that resides in the Macro Cell is a Rel-10 UE, Macro eNB10 designates two types of subframes on which CQI measurement should be performed, to Rel-10 UE30-4. As a result, Rel-10 UE30-4 calculates two types of CQIs, CQI on the subframes that receive weak interference, and CQI on the subframes that receive strong interference, from the Pico Cell, as the CQI of the subframes that Macro eNB10 transmitted to the macro Cell. As an example in which interference from the Pico Cell with the Macro Cell is strong, the case where UE30-4 that resides in the Macro Cell is located near the Pico Cell, the case where the transmission power of Pico eNB20 is high and others can be considered.
Specifically, in the example of FIG. 6, subframes #0, #2, #4, #6, #7 and #8 that Pico eNB20 transmitted to the Pico Cell are ABSs so that the interference that the Macro Cell receives from the Pico Cell is weak, while subframes #1, #3, #5 and #9 are normal subframes so that the interference that the Macro Cell receives from the Pico Cell is strong. In this case, Rel-10 UE30-4 calculates two types of CQIs, for subframes #0, #2, #4, #6, #7 and #8 and subframes #1, #3, #5 and #9 that Macro eNB10 transmitted to the Macro Cell. Here, subframes for performing CQI measurement, designated by Macro eNB10 to Rel-10 UE30-4 may also be part of subframes #1, #3, #5 and #9 and part of subframes #0, #2, #4, #6, #7 and #8. Rel-10 UE30-4 calculates two types of CQIs based on the designated two types of subframes.
In this case, Macro eNB10, using only the CQI value that is calculated based on subframes #0, #2, #4, #6, #7 and #8 that receive weak interference from the Pico Cell, or part of the subframes, among the two types of CQI values reported from Rel-10 UE30-4, can allocate the radio resources for downlink signals in Pico Cell's ABSs, that is to say subframes #0, #2, #4, #6, #7 and #8, to Rel-10 UE30-4.
With this arrangement, Macro eNB10 can allocate optimal radio resources for downlink signals to Rel-10 UE30-4 in the subframes that receive weak interference from the Pico Cell, based on the CQI value of the subframes,
On the other hand, when UE30-4 that resides in the Macro Cell is a Rel-8/9 UE, Macro eNB10 can only designate one type of subframes for performing CQI measurement, for Rel-8/9 UE30-4. As a result, Rel-8/9 UE30-4 calculates one type of CQI as the CQI of subframes that Macro eNB10 transmitted to the Macro Cell.
Specifically, in the example of FIG. 7, subframes #0, #2, #4, #6, #7 and #8 that Pico eNB20 transmitted to the Pico Cell are ABSs so that the Pico Cell's interference with the Macro Cell is weak, while subframes #1, #3, #5 and #9 are normal subframes so that the Pico Cell's interference with the Macro Cell is strong. Despite that fact, Rel-8/9 UE30-4 calculates only one type of CQI for all the subframes #0 to #9 without considering the difference in interference from the Pico Cell between the two types of subframes.
In this case, Macro eNB10 is unable to take into account the interference from the Pico Cell in allocating radio resources for downlink signals to Rel-8/9 UE30-4, and thus performs radio resource allocation for downlink signals to all the subframes using the single type of CQI value reported by Rel-8/9 UE30-4.
Herein, in general, the single type of CQI value reported by Rel-8/9 UE30-4 is calculated based on both the subframes in which the Macro Cell receives strong interference from the Pico Cell and the subframes in which the Macro Cell receives weak interference from the Pico Cell. Accordingly, the reported CQI value becomes smaller than that from a Rel-10 UE that resides at the same place because the Rel-10 UE performs calculation based on only the subframes in which the Macro Cell receives weak interference from the Pico Cell.
As a result, Macro eNB10 determines that the CQI of Rel-8/9 UE30-4, as to the subframes in which the Macro Cell receives weak interference from the Pico Cell, is inferior to the CQI of the Rel-10 UE that resides at the same place, and thus allocates in response to that CQI, a lower order Modulation Scheme or a lower order Coding Rate to Rel-8/9 UE30-4. Therefore, Rel-8/9 UE30-4 consumes more radio resources for Macro Cell downlink signals than Rel-10 UE that resides at the same place, hence causing a reduction of the capacity of the radio communication system.